can you pls tell me the logic for this code?

sd = 1;
      [M,N] = size(a1);
      r = imnoise2('rayleigh',M,N,0,1);
      figure;imshow(r),title('Noise to be added');
      c = find(r == 1);
      gp = a1;
      gp(c) = 255;
      r = uint8(r);
      figure;imshow(gp+r),title('Image after adding Multiplicative Rayleigh Noise');
      fp = imrest(gp,'chmean',3,3,-5.5);
      figure;imshow(fp),title('Image after removing Multiplicative Rayleigh Noise')